1) 'cz' (ISO 3166-1-alpha-2 you mentioned) is code for the Czech Republic, same as 'us' / 'gb' stands for the United States / the Great Britain.

2) 'cs' (ISO 639) is code for the Czech language, same as 'en' stands for the English language.

The same way we have 'en' (for the English language) or en_US/en_GB (for the English language as spoken in the United States/Great Britain) we should have 'cs' (for the Czech language) or 'cs_CZ' (for the Czech language as spoken in the Czech Republic, but this is redundant).